In general, in a power transmission part. of an automobile or the like, gears may be used for transmitting a driving force between two shafts. As the gears, there are known a helical gear whose tooth trace is helical, and a circular arc gear whose tooth trace is circular arc. As a prior art document, there is known JP-A-59-137661.
In the gear, a tooth contact thereof is one of important matters. If a desired tooth contact is achieved, generation of contact noise can be inhibited when gears are engaged with each other. In addition, if a desired tooth contact is achieved, stress at tooth flanks of gears can be controlled when the gears are engaged with each other, which improves durability of the gears.
However, in such a gear that a tooth trace of a tooth flank, which is to be engaged with another mating tooth flank, is curved with respect to an axial direction of the body of the gear, it has been difficult to achieve a desired tooth contact, because conventionally the tooth trace is continuously formed into a circular arc from a tooth root of the gear to a tooth tip thereof.